Client-Server synchronization on Client's shutdown

[modern-nm]

Hi, I am trying to make some feature and i have a problem that I cant solve for a long time. I cant make client to send a custom NetMessage to server for some post processing AFTER user hit "Close" button of ss14 window. I trying to send NetMessage on public override void Shutdown() of Content.Client\Entry\EntryPoint.cs. It is the most early point after "[INFO] game: Shutting down! Reason: Main window closed" i found to inject my code. So I call method _netManager.ClientSendMessage(msg); on Client side.
After that i expect that server capture my message OnSyncMessage(MyNetMessage msg) and process it, but i got [WARN] net: [::1]:59298: Received data message, but not connected. only (if server started in debug mode/ otherwise i dont recieve even that).
But.. if I place such bloody shit like System.Threading.Thread.Sleep(100); right after _netManager.ClientSendMessage(msg); magic is happened and my message delivered and processed. As i think, message is not delivered because client net status is Disconnected already. I cant find way to make netManager send it and dont die while message is flying to server

Help me, im under water

[PJB3005]

I cant make client to send a custom NetMessage to server for some post processing AFTER user hit "Close" button of ss14 window.

Uhm, why are you trying to do this?